In order to follow along, you'll need a version of Visual Studio from 2010 onwards, and one that includes unit testing (that doesn't include the Express editions, sorry). To demonstrate how to to write unit tests in Visual Studio, we're going to write a little program that tells us whether a year is a leap year or not.

First, let's create a new Console Application in Visual Studio 2010. You can do this by clicking on the "New Project..." link on the Start Page, or through the File Menu.

We can now implement the logic that will decide whether the year is a leap year or not. We can use the leap year algorithm on Wikipedia. The leap year calculation, although not quite complex, might seem oddly contrived if you're looking at it for the first time. If you're curious about why there are so many conditions, you might want to check out this question on StackOverflow.

The % is called a modulus operator - it performs a division but returns the remainder, not the result. So you know that year is divisible by 4 because the remainder is zero.

Right, now let's create some unit tests for our shiny new method. Before you run off and start belching out code, Visual Studio has this really neat feature where you can just right click somewhere empty and select "Create Unit Tests...":

In the screen that pops up, you can select the methods and properties for which you want to create unit tests. In our case it's just one method, but this is really useful if you have lots of code and want to generate a bunch of skeleton unit tests quickly.

In the "Output project" option at the bottom, you can choose whether to create a new test project or use an existing one. Again, in our case we can just create a new one, but if you already have a test project and want to add to it, you can just select it from the drop-down list. Anyway, hit "OK", name your unit tests project, and hit "Create". By convention, tests projects are usually named as [ProjectName].Tests .

Now, when you're done, you might get this message about InternalsVisibleTo:

See, this is because our Program class is not public, and that makes it internal by default. This means that the test project can't see it. Visual Studio allows projects to access non-public classes anyway by using this thing called InternalsVisibleTo, but in most cases I wouldn't recommend it. Just hit "No".

So now Visual Studio created a new project and some code for us:

In the test project, you'll notice that there's a reference to our LeapYearVs2010 project (because the tests need to use our actual code), and there's also a reference to this Microsoft.VisualStudio.QualityTools.UnitTestFramework thing (try fitting that in a business card) - that's needed for unit tests projects.

You'll also notice a bunch of code was generated. It's mostly full of crap, so let's just take out the bull and leave the beef:

using LeapYearVs2010;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System;

So, our usings include our LeapYearVs2010 project, and also a namespace for MSTest unit testing. Other than that, it's pretty straightforward: you have a test class (aptly marked with the [TestClass()] attribute, and it contains one or more unit tests (marked with the [TestMethod()] attribute). I know we haven't quite talked about attributes so far, but just stick them in the right places and don't worry about them.

Before we can write any proper unit test code, we'll need to make our Program class (in the LeapYearVs2010 project) public:

public class Program

We can now write our first unit test. At times you might find the autogenerated skeleton code for unit tests useful, but in our case you can just throw it out and replace it with this:

You can now run the unit tests via the Test menu, -> Run -> All Tests in Solution, or else by right clicking on a TestClass or TestMethod and selecting "Run Tests". This right click option is context sensitive, so if you Run Tests on a TestMethod it will run that unit test; if you Run Tests on a TestClass it will run all unit tests for that class.

Right, now at this point, if you also have Visual Studio 2012 installed, you might run into this bug where the tests will just kinda get stuck, like this:

Apparently this is a known issue and you'll need to install Service Pack 1 for Visual Studio 2010 to get things working. When they do, you'll see a nice green tick mark next to your test:

Whoops! Our second test failed. You can right click the test in the Test Results window and select "View Test Results Details" to obtain additional information including a stack trace (see screenshot above). If you need to, you can put a breakpoint in your test and then from the Test menu go to Debug -> Tests in Current Context while your cursor is in the unit test you want to debug. You can then debug the test as you would with normal code.

In our case, it's pretty clear that 1900 can't be a leap year because it's divisible by 100 but not by 400. So we need only update the expected value as follows:

bool expected = false;

Finally, let's take a look at how to create unit tests in Visual Studio 2012 and 2013. I'm going to create a new Console Application in this dark-themed VS2013 which looks like it came out of a chimney:

I'm going to set up Program just as I did in VS2010: make the class public, and add the method as before:

Now, let's create the unit tests. Right click in some empty space and...

Uhhhh... crap. Apparently the "Create Unit Tests..." option was removed in Visual Studio 2012, much to people's frustration. Fortunately, there are workarounds that can bring it back. If you don't want to do those, however, you're just going to have to create the test project and unit tests manually. To do this, right click on the solution, then Add -> New Project..., and from the Test category on the left, select "Unit Test Project". Give it a name and hit "OK".

This generates empty test code, and you can take off writing your unit tests from here. With VS2012, it's the same procedure.

Wow, this was long. Well, I hope it helped you learn about some of the unit testing tools that Visual Studio has to offer. It's very convenient because you can run and debug tests directly from within Visual Studio, and in Visual Studio 2010 there is the "Create Unit Tests" option which can be used to quickly generate skeleton code for unit tests of existing code. Sadly, this option is no longer there by default as from Visual Studio 2012, but there are ways of getting it back.